Phase modulation



July 28, 1936. G 1 USSELMAN v 2,049,143

PHASE MODULAT ION Filed April l, 1932 BY v ATTORNEY Patented July 28,1936 STATES PHASE MODULATION George L. Usselman, Port Jefferson, N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication April 1, 1932, serial No. 602,487

15 Claims.

This invention relates to a method of and means for impressing phasemodulations on radio waves or oscillations to be transmitted, and inparticular to an improved method of and means for accomplishing phasemodulation by the use of modulator tubes having output, as well asinput, electrodes connected in push-pull relation.

VI have found that for reasons possibly not fully comprehended at thepresent time phase modulated high frequency carriers are less subject tofading elfects than the same carrier modulated otherwise. Further, thatsatisfactory transmission of short waves which are phase modulated canbe carried on over distances heretofore impossible with the use of shortWaves modulated in amplitude or frequency.

Since phase modulated waves are less subject to the effects of fadingthan the same waves modulated otherwise and since phase modulated shortwaves can be transmitted over distances which cannot be covered by wavesmodulated otherwise, the practical application of the same is of thegreater importance. For example, the use of phase modulated signals indiversity reception enhances materially the quality of said reception.

The primary object of this invention is to provide an improvedmodulation method and system especially applicable to situations whereina two line balanced input and two line balanced output is required.

An advantage of a push-pull phase modulator as disclosed in the presentinvention is that neutralizing of the plate to grid capacities of theamplifying modulator may be accomplished more satisfactorily than it canbe done in phase modulators known heretofore in the art whether a singletube per stage is used or symmetrical tubes are used.

Briefly the above objects are attained in accordance with the presentinvention by the supplying of excitation voltages from a suitableoscillator to the grids of two modulator tubes through a phase changingdevice connected in series with each grid. In this arrangement theanodes of the two modulator tubes are connected to a common oscillatingtank circuit. This oscillating tank circuit can function at only onefrequency and phase at any given instant but the phase of oscillationsmay be shifted from time to time by giving one tube more power inputthan is given to the other tube. This is accomplished by changing thebias to the grids of the modulator tubes. The phase shift in themodulator tubes, as accomplished above, changes or modulates the phaseof the output frequency of the transmitter. In phase modulatedtransmitters constructed in accordance with the present invention thefrequency and the power output remain constant but the phase of theenergy is advanced or retarded relative to a normal constant frequencyabout an average phase angle. The rate at which the phase is advanced orretarded depends upon the frequency of the modulating potentials. Theamount of phase advance or retardation will depend upon the amplitude ofthe modulating potentials and also upon the amount of xed phase shiftcaused by the phase changing devices in the grid circuit.

The novel features of the invention have been pointed out withparticularity in the claims appended hereto. The method involved, thenature of the circuit, the operation thereof, and the numerousadvantages flowing therefrom will be better understood from thefollowing detailed description thereof and therefrom when read inconnection with the attached drawing in which:

Figure 1 illustrates the basic idea of the present invention; whileFigure 2 shows a modification of the arrangement of Figure 1.

Referring to Figure 1, A is a balanced type constant frequencyoscillator of any known type in use today, as, for example acrystalcontrolled oscillator of the type disclosed in Hansells United Statesapplication Serial No. 540,310, filed May 27, 1931, or any otheroscillator in use by radio engineers today, or a long line controlledoscillator such as disclosed by James L. Finch and James W. Conklin intheir United States application Serial No. 363,660, i'lled May 16, 1929,or any other long line frequency control generator known to radioengineers. The lines P and Q connect the oscillator output to a tunedgrid tank circuit H by way of capacities I and 2 respectively. The linesP and Q should be of substantially equal electrical length and similarin other characteristics relative to each other, to ground, etc., inorder that oscillations of substantially equal amplitude reach the tankcircuit H over each line from the generator A. The tank circuit Hcomprises a symmetrically tapped inductance 3 andV variable tuningcapacity Il. The center point of the tank circuit 3 is grounded througha blocking condenser W as shown. One terminal of the tank circuit H isconnected through a phase retarding means in the form of an inductance Ito the grid 6 of thermionic modulator tube 1, while the other terminalof the inductance 3 ofV circuit H is connected through a phase advancingmeans in the form of a capacity J to the grid 8 of a thermionicmodulator tube I0. The end of phase retarding inductance I, adjacent thegrid 6, is also connected to a resistance R1, the other terminal of R1is grounded for radio frequency by means of a blocking condenser Mconnecting the terminal of resistance R1 to the cathode I2 of tube 1.The terminal of the phase advancing capacity J adjacent the grid 8is'also connected to one terminal of resistance R2, the other terminalof R2 is connected by means of radio frequency by-passing condenser N tothe cathode I4 of tube I0. The low potential ends of resistorsRi and R2are also connected in phase opposition to the terminals of a tappedsecondary winding I6 of transformer T, the primary winding I8 of whichis connected to an audio frequency modulating source B as shown. Thecathode electrodes I2 and I4 of modulator tubes 'I and Iii respectivelyare connected tothe battery or power source 20 as shown. Ai constantdirect current biasing potential is applied through the secondarywinding I6 and resistances R1, R2 to the control electrodes 6 and 8 ofmodulator tubes 'I and I0 respectively by means of a connection I9between the negative terminal of'V the source 2l'and the center point onthe lsecondary winding I6 of transformer T.

.A tank circuit 22, comprising a capacity 2I and inductance 23, is tunedto normally oscillate at the frequencyof the wave or oscillations to bemodulated. The anodes 24, 26 of modulator tubes 'I and I0 respectivelyare connected in phase opposition. This is accomplished by connectingthe anodes 24 and 26, as shown, to opposite ends of the oscillationcircuit 22. The center point of the inductance 23 is connected to thepositive terminal of the source 20 so that a charging potential isapplied from the source 2E) to the anodes 24 and 26 respectively. Radiofrequency oscillations are shuntedaround the source 2i! from thefilament circuit by way of a radio frequency bypass condenser 2'Iconnected, asv shown in parallel with that portion of the sourcek 20included in the anode cathodecircuit of the modulator tubes 'I and I0`respectively. Radio frequencyA oscillations appearing in the tankcircuit 22 are prevented from passing through the source'20 by means ofa capacity 2'1. The potentials appearing on the anode 24 `of modulatortube "I are prevented from reacting on the grid electrode of tube 'I bymeans of a neutralizing capacity K connected between the anode 26 andthe grid 6 as shown, While, the potential'oscillation's appearing on theanode 26 of modulatortube I0 are prevented from reacting on the controlelectrode 8 of modulator tube I0 by means of a neutralizing capacity Lconnected between the anode 24 and the control electrode 8.

The phase modulated oscillations appearing in the tank circuit 22 may beutilized in any manner. For example, they may be fed to an amplierand/or frequency multiplier F by way of capacities U `and V in thebalanced lines connected to the output of the tank circuit C. The fre- Yquency multipliermay be of'any type, as, for example, as shown -inHansell, United States Patent No. 1,878,308; or Green, United StatesPatent No. 1,878,309; or any v other frequency -multiplier known toradio engineers. In general, pure phase modulation is accomplished bythis arrangement as disclosed. However, in the event thatthere has been.some Vamplitude modulation taking place,I Vthe balanced lines,`including. the condensers U and V may be caused to feed the energy tothe amplifier and/or multiplier F by way of an amplitude limiter deviceD. Since this amplitude limiter device D and amplifier or frequencymultiplier F per se form no part of the present invention, a detaileddescription thereof is thought unnecessary here.

In pointing out the operation of the device, it will rst be assumed thatoscillator A is supplying power at constant frequency to the input tankcircuit H. This power is supplied to the grid tank circuit H, throughthe balanced lines I and 2, and blocking condensers P and Q.Oscillations will be set upon in tank circuit H, the tuning or responseof which is determined by inductance 3 and capacity 4. Excitationvoltage at constant frequency will be supplied from the tank circuit Hthrough inductance I and condenser J to the grids 6 Yand 8 respectivelyof modulator tubes 'I and Il) respectively in substantial phaseopposition -or in push-pull fashion. However, the phase of the Voltageexcitation applied to the grid 6 of modulator tube 'I will be retardedto some extent by the inductance I, while the phase of the voltageexcitation applied to the grid 8 of modulator tube IIJ will be advancedto some extent by the capacity J. The amount of retardation andadvancement in the phase of the excitation applied tothe grids of thesetubes may be changed and will depend upon the retarding or inductive 0value given to the inductance I and the advancing or capacitive valuegiven to the condenser J, and also on the value ofthe resistance ofresistors R1 and R2 respectively.

The impedance in each side of the grid circuit H should be keptbalanced, then if both modulator tubes 'I and IB have the same directcurrent bias, which is the case when no modulation energy is beingsupplied from B, the power supplied byeachmodulator tube to the tunedtank circuit 22 will be equal and the potential oscillations in circuit22 will have a constant phase which will coincide with the average phaseposition of the excitation voltage, keeping in mind that theoscillations in circuit C are of constant frequency.

Now, if We assume that audio Vfrequency modulations are produced in Band applied through the transformer T to the control electrodes of 'Iand I0 respectively, the bias applied to the electrodes 6 and B of tubes'I and I0 respectively will vary in phase opposition and the tube havingthe lower bias at any particular instance will supply the most power tothe tuned tank circuit 22. Since the excitation to the grids 6 and 8 ofthe two modulator tubes l and Il] respectively has a phase differencethe power supplied to the tank circuit 22, by the anodes 24, 26 of thetwo modulator tubes 'I and I0, will have a phase difference. The phaseof the oscillations appearing in circuit 22 will change or approach thatof the tube having the lower grid bias and therefore supplying the morepower. The amount of phase change in the circuit 22 lwill besubstantially proportional to the difference in power delivered by thetubes 'I and I0 to the circuit 22. It is obvious, however, that theremay be no greater phase shift in the tank circuit 22 than there is phasedifference of excitation applied to the grids of tubes 'I and I0respectively.

From the above description it can be seen that variations in audiofrequency amplitude applied from source B to the control electrodes ofthe modulator tubes 1 and I0 respectively will pronormal constantfrequency oscillations in circuit 22.

One advantage of the circuit as disclosedabove is that the tubes, beingconnected in push-pull,

may be readily neutralized bymeans of the capacities K and L.

This circuit also provides all of the advantages attendant on the use ofpush-pull tubes since the plates or anodes are connected so that theydeliver power in push-pull fashion and in phase opposition.

This circuit also readily lends itself to grid neutralization and toother schemes of balancing applied to radio frequency circuits.

In Figure 2 is shown a modication of the arrangement of Figure 1. Inthis arrangement, which is otherwise similar to the arrangement shown inFigure 1, screen grid tubes are used, thereby eliminating` the necessityof neutralizing the inter-electrode capacity as was the case in thearrangement of Fig. l. This arrangement also differs from thearrangement shown in Figure 1 in that the audio frequency modulation isintroduced, as shown, on the screen grid electrodes of the modulatortubes instead of on the the present arrangement are amplified beforebe-V ing used to phase modulate the carrier.

The oscillation generator A of Figure 2, and the means by whichoscillations at a carrier frequency, the phase of which are alternatelyadvanced or retarded an amount determined by the inductance I andcapacity J, are caused to appear in the tank circuit 22, is the same asthe corresponding elements and means of the arrangement of Figure 1. Themeans by which the phase modulations are impressed on the carrier in thearrangement of Figure 2, however, differ.

In Figure 2' the tubes 1 and i9 are ofvth screen grid type and themodulating frequencies are impressed on the screen grid electrodes 36and 31 respectively to thereby vary the conductivity of the tubes 1 andI9 respectively, and in that manner modulate the phase of the carrier`oscillations at a rate determined by the frequency of the modulatingpotentials and by an amount determined by the amplitude of themodulating potentials limited,Y of course, by the alternating phaseadvancing and retarding effect imparted to the carrier frequencyoscillations by the inductance I and capacity J. The modulatingpotentials are applied to screen grid electrodes 36, 31 from the anodes34, 35 respectively of'a pair of thermionic tubes 28, 29 having theiranodes connected in parallel through resistances R3, R4 to a point on apotentiometer 38 in parallel with source 29. The modulating potentialsobtained from the source B are impressed on charging potentials by wayof resistances R3, R4V respectively, which also supply charging poten-Vtial for the anodes 34, 35 of the tubes 28 and 29 respectively. Biasingpotential for the tubes 28 and 29 is supplied by way of a lead I9connected with the source 29 and the midpoint of winding I6. Biasingpotential for the grid electrodes of the tubes 1 and I ilV respectivelyissupplied by way of 'a lead I9 connecting the resistances R1, Rz to apoint on the potentiometer 39 connected in parallel'with the source 20.

In operation the modulating frequency potentials are applied in phaseopposition to'the control electrodes A32 and 33, of tubes 28 and Z9respectively. These tubes become alternately more conducting and lessconducting at a degree and-frequency determined by the intensity andfrequency of the modulating potentials. When tube 28 becomes moreconductive the anode circuit thereof, including the resistance R3, drawsmore current, which causes an increase in voltage drop across resistorR3.' The voltage drop across Y a resistor opposes the potential whichcauses the current to flow, so that an increase in current throughresistor R3 decreases the positive potential on the screen gridelectrode 31. This potential variation applied to screen grid 31 governsthe amount of energy supplied to the tank circuit 22 connected with theanode 26 of tube IU. f In this case the energy supplied by tube i9 isreduced. At the same time, since the modulating potentials are appliedin phase opposition to the control electrodesof tubes 28 and 29, tube 29is rendered less conductive so that less current ows in resistance R4.This causes less voltage dro-p in resistance R4, which results in anincrease of positive potential applied to the screen grid electrode 36of tube 1, therebycausing the anode 24 of tube 1 to draw more current'and to supply more energy to the tank circuit 22. Since the energy intank circuit 22 is mostly from tube 1, the phase of the energy suppliedfrom tube 1 to a great extent determines the phase of the energy in thetank circuit.V When the modulating potentials applied to-the secondarywindings I6 reverse in sign the reversed operation takes place. Tube '29now draws more current while tube 28 draws less current so that thepositive potential applied to the screen grid electrode v36 decreasesand the positive potential applied to the screen grid electrode 31increases. Tube 1 now becomes less conductive and tube I9 becomes moreconductive. The energy in the tank circuit 22, therefore, is mostly.energy supplied from the anode 26 of tube i9. For that reason the phaseof the energy intank circuit 22, at this particular instant, isdetermined to a great extent by the phase of the energy supplied fromthe tube I9. In this manner the high frequency oscillations suppliedfrom A, through high frequency phase shifting means I and J to tubes 1and I9 respectively, are modulated in phase at a degree and rate`determined by the intensity and frequency of the modulatingpotentialsproduced in B. As will be obvious, the object of this arrangement,except as otherwise vindicated above, is thesame as the object of thearrangement shownin Figure l.

The present arrangement includes all of the4 advantages of thearrangement of Figure 1 and, in addition thereto, eliminates thenecessity of the use of neutralizing capacities to prevent reactionbetween input and output circuits of the modulator tubes. Y

The amount of desired sideband energy and the amount of undesiredsideband energy, resulting from the phase modulation of the carrier atsignal frequency accomplished inthe devices of Figures 1 and 2, dependsin part on the amplitude of thecarrier orof the modulating potentials.By regulating the amplitude of either the carrier or the modulatingpotentials, the amount of the undesired sideband or ,components may bediminished or increased at will` By regulating the amplitude ofi themodulating Vpotentials at the source B in either of the arrangementsshown in VFigure- 1 or 2, or by adjusting the lead I9 along thepotentiometer 39 of Figure 2 to control the amplification of themodulation frequency amplifiers 28 and19, the amplitude of themodulating potentials `acting on the impedances of tubes 'l and lllmaybe regulated, thereby controlling the amount of desired and undesiredsideband energy appearing in the circuit 22. The amplitude componentsand other undesired energy of the same nature may be removed in the unitD. The phase modulation components may be increased in degree bymultiplying the frequency of the phase modulated energy in F.

Having thus described my inventionand the operation thereof, what Iclaim is:

1. Transmitting means comprising a source of oscillations, a pair ofthermionic tubes of the screen grid type, connections between saidsource and the control electrodes of said tubes for applyingoscillations in phase opposition to the control electrodes of saidtubes, means for phase modulating said oscillations comprising phaseshifting means in each of said connections, a tank circuit connectingthe .anodes of said tubes in push-pull relation, a source of modulatingpotentials, and means for impressing the modulating potentials in phaseopposition on the screen vgrid electrodes of said tubes.

2. Means for amplifying and phase modulating high frequency energycomprising, a pair of thermionic tubes of the screen grid type havingtheir input electrodes energized in phase opposition by carrierfrequency` potentials, means for retarding the phase of the carrier`frequency potentials applied to'one of said grid electrodes, and meansfor advancing the phase of the carrier frequency potentials applied tothe other cf said grid electrodes, a tank circuit connected between theanodes of said tubes, and means-for impressing modulating potentials inphase opposition on the screen grid electrodes of said tubes.

3. An arrangement as claimed in claim 2 in which said last named meansincludes amplifying means.

4. Means for impressing phase Amodulations at signal frequency on highfrequency oscillations comprising, a pair of thermionic tubes of thescreen grid type, means for impressing high frequency oscillations inopposition on the control electrodes of said tubes, phase shifting meansinterposed between the control electrodes of one of said tubes 'and saidimpressing means, a load circuit connected between the anodes of saidtubes, a pairv of thermionic amplifiers, means for impressing modulatingfrequencies on the control electrodes of said amplifiers in phaseopposition, a connection between the anode of each of said amplifiersand the Screen grid in one of said screen grid tubes, and means forconnecting the andoes of said last named tubes to aV source ofpotential.

5. A means for producing and phase modulating high frequencyYoscillations including the combination of a constant frequencyoscillator, a push-pull modulator having =a tank circuit which may beconnected with the input electrodes thereof and a tank circuit connectedwith the output electrodes thereof, a balanced line connecting saidoscillator to said input tank circuit, phase shifting Vmeans ofdifieren-t characteristics .connected between 'the 4co'n-tr'ol?electrode'of each tuned tank circuit connected in push-pull rela-v tion with thecontrol electrodes thereof and a tank circuit connected in push-pullrelation with the output electrodes thereof, means for connecting saidoscillation generator to said input tank circuit, phase shifting meansof different characteristics connected between the control electrodeofeach of said modulator tubes and a terminal of said input tank circuit,means for applying direct current biasing potential to the controlelectrodes of each of said modulator tubes, and abalanced means fordifferentially applying modulating potentials to the control electrodesof said tubes.

'7. The combination with a source of carrier frequency oscillations of aphase modulator comprising, a pair of thermionic tubes having theircontrol grids connected by reactances of different electricalcharacteristic to said source of oscillations, their anodes connected inpush-pull relation by an output circuit `and their anode to' controlgrid capacities neutralized by variable capacities, a source ofmodulating potentials connected by way of thermionic repeaters in phaseopposition to the impedances of the tubes in said modulatorwherebycarrier oscillations from said source are modulated in phase atsignal frequency and inadvertently modulated in amplitude, an amplitudelimiter connected with the output circuit of said tubes whereby theamplitude modulation inadvertently accomplished is removed from themodulated energy, and a frequency multiplier coupled with said limiterto increase the degree of phase modulation accompli-shed.

8. The combination of an oscillator and a tubes each having control gridand anode electrodes, said control grids and said anodes being coupledin push-pull relation by input and output circuits respectively, each ofwhich circuits include an nductance, a balanced line connecting theoscillator in parallel with a portion of the inductance in said inputcircuit, phase shifting reactances of different character in said inputcircuit connected with the control grid of each of said tubes in saidmodulator stage, a balanced line connecting'a portion of the inductancein the output circuit of said tubes to said load circuit, and means forphase modulating the oscillations `from said oscillator comprising meansfor applying modulating potentials in phase opposition to similarelectrodes in each of said tubes.

V9. Transmitting means comprising the combination of, a constantfrequency oscillation generator, a pair of modulator tubes each havingcontrol grid a-nd anode electrodes, an input circuit, phase shiftingreactances of different character connecting the control grids of saidmodulator tubes in push-pull relation by waykof said input circuit, anoutput circuit connecting the anodes' potentials to the control gridelectrodes oi each of said tubes and energizing potentials to the anodesof each of said tubes, and means for phase modulating the oscillationsfrom said oscillation generator comprising a balanced means fordifferentially applying modulating potentials to like electrodes in eachof said tubes.

10. A phase modulating means comprising, a pair of thermionic tubes eachhaving anode and control electrodes, a tank circuit adapted to beenergized by carrier frequency oscillations, a reactance connecting oneterminal of said tank circuit to the control electrodes of one of saidtubes, a reactance connecting the other terminal of said tank circuit tothe control electrode of the other of said tubes to excite the same inout of phase relation, a circuit connecting the anodes of said tubes inpush-pull relation, a source of modulating potentials, and a circuit forapplying said potentials from said last named source in phase oppositionto like electrodes in each of said tubes.

11. The combination of a constant frequency oscillator, a modulatorstage of the thermionic tube type having input electrodes and outputelectrodes connected in push-pull circuits each of which includes aninductance, and a load circuit, a balanced line connecting saidoscillator to points on the inductance connected with the inputelectrodes of said tubes, a balanced line connecting points on theinductance connected with the output electrodes of said modulator tubesto said load circuit, phase shifting means connected with the inputelectrode of each of said modulator tubes, said phase shifting meansbeing of different character, a source of modulating potentials, and acircuit for applying modulating potentials from said source in phaseopposition to like electrodes in each of said tubes.

12. The combination of a constant frequency oscillator, a modulatorstage including, thermionic tubes connected in push-pull relation byinput and output circuits each circuit including an inductance, and aload circuit, a balanced line connecting said oscillator to points onthe inductance in the input circuit of said modulator stage, a balancedline connecting points on the inductance in the output circuit of saidmodulator stage to said load circuit, an inductive reactance connectedbetween the inductance in said input circuit and the input electrode ofone of said tubes in said stage, a capacitive reactance connectedbetween the inductance in said input circuit and the input electrode inthe other of said tubes in said modulator stage, and a circuit forapplying modulating voltages in phase opposition to the impedancebetween two electrodes of each tube in said modulator stage.

13. The combination of a constant frequency oscillator, a pair ofthermionic tubes each having an anode, a cathode and a controlelectrode, reactances connecting said control electrodes and said anodesin push-pull circuits, a load circuit, a balanced line connecting saidoscillator to the reactance connected with the control electrodes ofsaid tubes, a connection between the electrical center of saidlast-named reactance and the cathodes of said tubes, a balanced lineconnecting the reactance between said anodes to said load circuit, aconnection between the electrical center of said last-named reactanceand the cathodes of said tubes, phase shifting reactances connected withthe control electrode of each of said tubes, said phase shiftingreactances being of different character, and a circuit for applyingmodulating voltages in phase opposition to like electrodes of each ofsaid tubes.

14. Means for impressing phase modulations at signal frequency on highfrequency oscillations comprising, a pair of thermionic tubes of thescreen grid type, each having anode, cathode, control grid and screengrid electrodes, a circuit for impressing high frequency oscillations inphase opposition on the control grids of said tubes, phase shiftingreactances interposed between the control grid of each of said tubes anda different point on said impressing circuit, a load circuit connectedwith the anodes of said tubes, a pair of thermionic amplifier tubes eachhaving an anode and a control grid, circuits connected with the controlgrids of said amplier tubes for impressing modulating potentials inphase opposition on the control grids of said amplifier tubes, andconnections between the anode of one of said amplier tubes and thescreen grid in one of said screen grid tubes and between the anode ofthe other of said amplifier tubes and the screen in the other of saidscreen grid tubes.

15. In a phase modulating system, a pair of thermionic tubes each havinga control grid and a cathode, a tank circuit adapted to be energized byhigh frequency oscillatory energy, an inductive reactance connecting oneterminal of said tank circuit to the control grid of one of said tubes,a capacitive reactance connecting the other terminal of said tankcircuit to the control grid of the other of said tubes, a connectionbetween a point on said tank circuit and the cathodes of said tubeswhereby said control grids are excited substantially in phase oppositionby oscillatory energy in said tank circuit, and a source of modulatingvoltages connecting the control grids of said tubes in phase opposition.

GEORGE L. USSELMAN.

